1. Field of the Invention
The invention relates to an apparatus for recording an electric signal on a magnetic record carrier in tracks which are inclined relative to the longitudinal direction of said record carrier, and to an apparatus for reproducing a signal recorded by means of such a recording apparatus.
2. Description of the Related Art
Corresponding to U.S. Pat. No. 4,757,392, discloses European Patent Application 210,773 a recording apparatus as defined in the opening paragraph, comprising:
- an input terminal for receiving the electric signal; PA1 - a signal separator, having an input coupled to the input terminal for dividing the electric signal into consecutive blocks having a specific length of time, and for applying the consecutive blocks to a first and a second output in such a way that blocks having odd sequence numbers are applied to the first output and blocks having even sequence numbers are applied to the second output; PA1 - a time-base correction circuit which is constructed to provide time compression or time expansion of the consecutive blocks, to delay blocks having odd sequence numbers relative to those having even sequence numbers, and to supply the two signals thus processed to a first and a second output, respectively; PA1 - at least one pair of write heads having different azimuth angles and arranged on a rotatable head drum, one write head of a pair being arranged to be coupled to the first output of the time-base correction circuit and the other write head of the same pair being arranged to be coupled to the second output of the time-base correction circuit. PA1 - at least one pair of read heads having different azimuth angles, and arranged on a rotatable head drum; PA1 - a time-base correction circuit having a first and a second input arranged to be coupled, respectively, to one read head and to the other read head of the pair of read heads, the correction circuit being constructed to provide a time compression or time expansion of the signal blocks applied to the first and the second input respectively, to delay the signal blocks applied to one input relative to those applied to the other input, PA1 - a signal-combination unit having a first and a second input and an output, for combining the signal blocks applied to the first and the second input in order to restore the electric signal and for feeding the electric signal to the output, which output is coupled to an output terminal for supplying the electric signal.
The two write heads of a pair of write heads are regularly spaced along the drum circumference. The known apparatus can also be used as a reproducing apparatus, in which case it comprises:
The known apparatus has the disadvantage that during operation it produces a high acoustic noise level, that only very dimensional tolerances of the components are permissible, and that sometimes the reproduction quality is not satisfactory.
The invention aims at mitigating these drawbacks and therefore proposes an apparatus for recording an electric signal which is characterized in that the write heads of a pair of write heads are are arranged close to each other and have a mechanically rigid coupling to each other, and in that the time base correction circuit is adapted to provide a time expansion or time compression of the signal blocks by a factor of .alpha.*n/(180*(M+1), where .alpha. is the wrapping angle of the record carrier around the head drum and differs from 180.degree., n is the number of head pairs, and M is the number of times within a specific time interval that a head pair which comes in contact with the record carrier during said time interval does not record a signal in the record carrier, this time interval being defined by those instants at which two consecutive track pairs are recorded by one or two head pairs. The apparatus for reproducing an electric signal is characterized in that the read heads of one pair of read heads are arranged close to each other and have a mechanically rigid coupling to each other, and in that the time-base correction circuit is adapted to provide a time compression or time expansion of the signal blocks applied to the first and the second input by a factor of 180*(M+1)/(.alpha.*n), where .alpha. is the wrapping angle of the record carrier around the head drum and differs from 180.degree., n is the number of head pairs and M is the number of times within a specific time interval that a head pair which comes in contact with the record carrier during said time interval does not read a signal from the record carrier, the two track pairs being read consecutively by one or two head pairs.
As the heads of one pair of read or write heads are arranged at one location on the head drum, these heads hit against the record carrier only once every revolution of the head drum during recording and reproduction which enables the acoustic noise level to be reduced. In the case of head-level variations which manifest themselves similarly during every revolution of the head drum, for example as a result of wobbling or hunting of the head drum, the tracks written by the two heads of one pair of heads are yet recorded parallel to each other on the record carrier. The tracks may then be warped. However, if the apparatus comprises positioning means for positioning the head pair in a direction transverse to the track, said tracks can still be read correctly. Moreover, if the positioning means are constructed as a dynamic tracking system, only one actuator is needed, on which the pair of heads is arranged.
In the known apparatus, in which the heads of a pair of write or read heads are spaced at 180.degree. from each other along the head drum circumference and which comprises a dynamic tracking system, "false lock" may occur. This means that tracks are sometimes recorded or read in a wrong sequence. This is because the heads are not disposed at the same level relative to each other. Said false lock problem does not occur in the apparatus in,accordance with the invention because the heads of a pair of write or read heads are now arranged close to each other and have a rigid mechanical coupling to each other, said heads being arranged, for example, on one actuator. Consequently a control system for maintaining the heads at the same level, as is required in the known apparatus, is not needed now.
In a first embodiment n=1 and M=0. This means that one head pair is arranged on the head drum and records or reads one track pair during every revolution of the head drum. If the wrapping angle is smaller than 180.degree. the correction circuit in the recording apparatus should provide a time compression of the signal by a factor of .alpha./180 and, consequently, the correction circuit in the reproducing apparatus should provide a time expansion of the signal by a factor of 180/.alpha.. If the wrapping angle is larger than 180.degree. the correction circuit in the recording apparatus should provide a time expansion of the signal and the correction circuit in the reproducing apparatus should provide a time compression of the signal.
In another embodiment n=1 and M=1. This means that only one head pair is arranged on the head drum. This head pair writes or reads one track pair during every two revolutions of the head drum. During recording the signal should then be time-compressed by a factor of .alpha./360 and during reproduction the signal must be time-expanded by a factor of 360/.alpha..
The apparatus may be characterized further in that it comprises a second pair of write or read heads having different azimuth angles and arranged on the rotatable head drum, said second pair of heads being arranged close to each other and having a mechanically rigid coupling to each other.
An apparatus comprising two or more head pairs which are equidistantly spaced along the circumference enables information to be recorded or read at different tape speeds but with a constant speed of rotation of the head drum.
An apparatus comprising two head pairs which are 180.degree. spaced apart on the head drum can record at the "normal" tape speed and at a tape speed which is twice as high. In the first case, only one pair of heads records adjoining tracks on the record carrier. For the parameters n and M this means that n=2 and M=1. At a tape speed which is twice as high, a spacing will be obtained between two tracks recorded directly after each other by one pair of heads, in which spacing the other pair of heads can record exactly one track. At this speed both pairs of heads consequently record tracks on the record carrier. For the parameters n and M, this means that n=2 and M=0.
Obviously, the same applies during reproduction. If during recording the tracks have been recorded at the normal tape speed only one pair of read heads is used for reading the information during reproduction. If during recording the tracks have been recorded at twice the normal tape speed, both pairs of read heads are used during reproduction, the tape speed being equal to that during recording.
The information recorded by the second pair of write heads may comprise additional information, for example the finest detail of an encoded video picture or an entirely different signal, or it enables a higher resolution standard to be adopted.
Recording or reading information at different tape speeds and constant speed of rotation of the head drum is also possible in the case of more than two pairs of write or read heads. For example, in the case of three pairs of heads writing and reading at the normal tape speed is effected with one write head or read head (n=3 and M=2), while at three times the speed, all the three heads are operative (n=3 and M=0).
It is to be noted that recording and reproducing a digital video signal by means of at least one pair of heads which are arranged close to each other and which have a mechanically rigid coupling with each other is known and is described in the publication "An experimental digital video recording system" by Driessen et al in IEEE Trans. on CE, Vol. CE-32, No. 3, August 1986, pp. 362-70. However, in this system, the video signal is not time compressed or expanded. Neither is one signal component delayed in time relative to the other.